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The function of obtaining an IPv6 address?

unsigned char networkMask [sizeof (struct in6_addr)] = { [0 ... (sizeof (struct in6_addr) - 1)] = 0xff };

what is portrayed here (0...(sizeof)). How is this array distributed?

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This particular syntax is a GCC extension designated by the initializer . With it, you can initialize the array as follows:

unsigned char foo[<n>] = { [0 ... <n> - 1] = <k> };

Thus, <n>is the number of members, and <k>is any given value of the member.

In the code you specified, it initializes an array networkMaskwith 0xfffor elements from the index 0through sizeof(struct in6_addr) - 1. In other words, it initializes an array of size struct in6_addrand sets all bits to 1.

This would be equivalent to this, given that the IPv6 address is 16 bytes:

unsigned char networkMask[sizeof(struct in6_addr)] = { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255 };
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